An optical network is usually composed of a plurality of nodes connected together by lengths of optical fibre known as ‘spans’. A typical optical network is depicted schematically in FIG. 1. The network 50 comprises a plurality of nodes 20 connected by fibre spans 30. Only three nodes 20 are shown for clarity. However, it will be appreciated that a network will usually contain many more nodes. Each span may comprise two (or more) fibres, for example an outbound fibre to carry communications traffic (in the form of a modulated optical signal) away from a node, and a return fibre to carry traffic towards the node. Although the spans shown connect two adjacent nodes, it will be appreciated that “span” may equally refer to a connection between two non-adjacent nodes, where there are intervening nodes.
The network 50 may contain many different types of nodes, each having one or more functions. For example, nodes 20a and 20c are add/drop nodes, at which optical traffic signals are added or dropped from the network 50. Such nodes comprise laser optical radiation sources onto the output of which an electrical signal is modulated to create a traffic signal, as well as demodulation equipment able to recover information in the form of an electrical signal from received optical radiation. Node 20b is a repeater node, which does not add or drop signals from the network 50, but instead forwards received optical signals on, usually with additional amplification to boost the signal power. Other nodes may perform both those node functions, and may be able to introduce/remove signals from the traffic as well as amplify and retransmit received traffic signals.
The transmission of signals throughout the network is controlled from a central office 40. The central office 40 oversees signal routing and fault detection within the network.
When an optical fibre within the network breaks, it is usually possible to detect the span in which the break has occurred from the fact that a receiver node/amplifier does not receive any optical power from a remote transmitter node (sometimes also termed a launch amplifier). When the term ‘break’ is used, it is meant that the fibre is damaged such that optical signals are not transmitted all the way along the fibre to a receiver remote from the source of optical signals.
Each span within a network can be many kilometres long (usually up to 100 km). Thus, when a fibre break occurs, it is desirable to detect the position of the fibre break along the span with a suitable precision, in order to give the personnel that must repair the fault as precise information as possible about the position of the break, to avoid the need to search the entire span for the break. However, although as discussed above it is quite simple to determine which fibre span is damaged, it is not easy to detect exactly where the fibre is broken.
The most common prior art methods for locating fibre breaks launch optical power into an end of the affected span and then process the backscattered signal. Some methods use an OTDR (Optical Time Domain Reflectometer) integrated in different ways into the communications system, or used as a stand-alone instrument operated by personnel. Optical time division reflectometry requires sending high power short duration pulses of radiation into a fibre under test, and detecting the signal that is backscattered from a break or defect in the fibre. The distance to the break can be determined by processing the signal in a known way.
Such OTDR methods can be divided into methods suitable for in-service systems, and methods which are only suitable for out-of-service systems. For example, consider a system which comprises an add/drop node shortly after a launch amplifier, where there is a break after the add/drop node. Traffic dropped at the add drop node will not be affected by the break, but may be affected by any diagnostic test run from the launch amplifier which tries to locate the break. In the case of an in-service test, the dropped traffic would be (substantially) unaffected by the test. In the case of an out-of-service test, however, the dropped traffic might be severely disrupted, and the add/drop node and other equipment further downstream might be damaged, if traffic is not halted whilst the test is performed.
In the case of out-of-service systems, an OTDR signal is transmitted into the affected fibre link at the wavelengths of the traffic. This signal is backscattered from a break of the fibre, and then detected and processed in a suitable way. Such systems can only be used when the entire link, in both directions, is considered out of service, because the use of an in-band signal (i.e. a signal at the same, or similar, wavelength to the traffic) may interfere with and disrupt traffic present on the span in question, and on downstream spans. Care must be taken to avoid burning out the receiver optics with the OTDR signal, because the in band OTDR signal is amplified by optical repeaters in the same way as traffic channels, and so risks being demultiplexed and sent to the expensive and delicate receiver optics.
In-service systems overcome these problems by using a test signal at a different wavelength to the traffic, for example an out-of-band tone, or the optical supervisory channel, for the OTDR (so that such a signal will not be amplified or demultiplexed). However, such systems are more expensive, as they require additional equipment to conduct the fibre testing, in the form of an additional launch amplifier, and occasionally even an additional ‘spare’ fibre.
The requirement for expensive additional equipment to be installed at each node can be overcome by the use of a stand-alone OTDR instrument. However, this has the drawback that it requires the use of human personnel, which must first detect the location of the break from one end of the fibre, the move to the site of the fibre break in order to repair the break, resulting in a higher labour cost, and the possibility of further failures due to the fact the personnel must handle the equipment inside the station/node of the link).
It is an object of the present invention to alleviate some or all of the above problems.